Transcript
ECET 3410
Lab 2: TIME DOMAIN REFLECTOMETRY
This exercise investigates the concepts of time domain reflectometry. A TDR is the instrument used to observe wave phenomena from a connection point on a transmission line. This connection point is typically one end of a line section. Instruments that are available commercially provide internal sources for line excitation and offer a variety of features that aid in determining locations and characteristics of faults, mis-terminations, line loss, propagation velocity, and line length. These instruments are available for both metallic and non-metallic (fiberoptic) lines. Typical TDRs employ a 150-ps rise-time step voltage and can locate faults within a distance of a few centimeters. A reflection occurs each time the step encounters a discontinuity. The reflection is added to the incident wave and is displayed on the scope output screen. The time required for the reflection to return to the oscilloscope locates the discontinuity, indicating the two-way propagation time. The shape and magnitude of the reflected wave indicate the nature and value of the mismatch. The mismatch can be resistive, inductive, or capacitive. An inductive discontinuity reflects a voltage spike having the same polarity as the incident step and appears as shown below.
A capacitive discontinuity reflects a voltage spike of opposite polarity, producing the display below.
In the lab you will investigate the effect various resistive mismatches produce when measured using a TDR. The sign and magnitude of the signal reflected from the end of the cable back to the TDR is dependent on the impedance of the line and the terminating impedance. The ratio of the reverse traveling signal to forward traveling signal at the “load” end of the cable defines the reflection coefficient of the terminating impedance. The following equations can be used to determine the reflection coefficient: ΓL = V- / V+ = I- / I+ = (ZL - Z0) / (ZL + Z0) The reflected signal (V-) is equal to the reflection coefficient (ΓL) times the forward signal (V+). A resistive discontinuity having a value greater than the line impedance results in a positive reflection coefficient such that a step of the same polarity is displayed after the signal travels twice the length of the cable. A resistive discontinuity having a value less than the line impedance results in a negative reflection coefficient and a step of the opposite polarity. The magnitude of the step is determined by the degree of impedance mismatch.
TEST PROCEDURE Instrumentation TEK 11801 Digital Sampling Oscilloscope with SD-24 TDR/Sampling Head Device(s) Under Test [DUT] 12’ and “long” sections of RG-58/U coaxial cable used during Lab 1 Reel of RG-58 A/U coaxial cable Reel of RG-62/U coaxial cable Coaxial terminations (short circuit, 50Ω, 75Ω, 100Ω, and 200Ω) CAUTION: DO NOT REMOVE SAMPLING HEAD. properly, see your lab instructor.
If TDR does not appear to function
1. Remove SMA short circuit termination from Channel 1 of Sampling Head. 2. Using an SMA-to-BNC adaptor, connect the 12’ section RG-58/U cable used during Lab 1. Leave the other end unterminated. 3. Turn on and allow TDR to performance self test. 4. Select "Waveform" function (button to the right of the display). 5. Using the touch-screen menu, select “Graticule”. Set up the vertical display for voltage and the horizontal display for length measurements. Adjust either vernier control for the appropriate velocity factor. Choose “Feet” for measurement units. Then press “Exit” to leave this menu. 6. Press the “↔” symbol at the top of the screen to adjust the horizontal magnification. Use the upper vernier to adjust the magnification so that the entire cable length is displayed. You can adjust the horizontal position of the trace using the lower vernier. 7. Press the vertical “↔” symbol at the left side of the screen to adjust the vertical magnification. Use the upper vernier to adjust the vertical magnification to maximize the size of the vertical displacement of the trace. The other end of the cable should remain unterminated. You can adjust the vertical position of the trace using the lower vernier. 8. Select the "Cursor" function and use vernier controls to determine cable length in feet. The length appears as ∆f/2 (half the total distance traveled by the pulse) in the bottom center of the screen. 9. Measure the length of the “long” cable and accurately sketch the TDR display. Be sure to label all relevant voltage levels and distances on the sketch, as well as the load type (unterminated). 10. Replace the 12’ cable with the “long” cable also used during Lab 1. Leave the other end unterminated. Utilizing the same steps applied to the 12’ cable, measure the length of the “long” cable and accurately sketch the display.
11. Place the short circuit terminator on the cable and accurately sketch the display. Be sure to note the final voltage level on your sketch. 12. Replace terminator with a 100Ω terminator on the cable and accurately sketch the display. 13. Repeat step 12 using a 200Ω, a 75Ω and a 50Ω load. Accurately sketch each display. 14. Utilizing the same procedure, determine the length of a reel (provided by your instructor) of RG58 A/U coaxial cable in feet. Accurately sketch the display. Also, determine what happens when a 50Ω load is placed on the end of the cable. (Be sure to account for the nominal velocity of propagation for this cable type.) 15 Again utilizing the same procedure, determine the length of a reel (provided by your instructor) of RG-62/U coaxial cable in feet. Accurately sketch the display. Also, determine what happens when this cable is terminated with a 50Ω load? 16. When cables with different velocity factors are used together, the TDR can still be used to verify cable lengths. This is done by setting the TDR velocity factor setting to 1.0 and multiplying the measured length of each section of cable by its velocity factor. Connect the long section of RG58/U cable from Lab 1 directly to the TDR and then connect the RG-62 A/U to this section of cable. Measure the length of each cable section and then use the cable velocity factors to calculate the actual cable lengths. Be sure to accurately sketch the display for your laboratory report. 17. When you have concluded your testing, disconnect the SMA-to-BNC adaptor. Replace the SMA short circuit on the Sampling Head and turn the TDR off.
TEST EVALUATIONS 1. Tabulate the lengths of the various unterminated sections of coaxial cable tested (steps 9, 10, 14 and 15). Numerically compare the calculated lengths with the lengths indicated on the cables or reels. 2. Using the measured data from steps 12 and 13, determine the reflection coefficients for each load and compare them to expected (theoretical) reflection coefficients. Neatly tabulate your results. Discuss the reasons for the differences between the measured and calculated values. 3. Assuming total wave reflection at the short-circuit termination (step 11), determine the approximate attenuation for the section of RG-58/U coaxial cable. This can be done using the following identity: Γin = V-/V+= +⎪ΓLoad⎪e-2αl and solving for the attenuation, αl. The solved value “αl” is in units of Nepers and must be converted to decibels by the factor 8.686 dB/Np. Compare the results to those you measured during Lab 1. Based on this comparison, discuss the frequency content of the signal produced by the TDR? Justify your results. 4. Describe a test technique for determining the relative permitivity, εr, of an unmarked section of cable. Be specific to describe all measurements, and provide formulas for all calculations. 5. Discuss both the strengths and the limitations of time domain reflectometry. Be sure to specifically address using TDR as a technique for length calculations.
